using the apple hardware test
Lion was downloaded from app store so I have not install disk and option D does not bring hardware test app as the support document suggests.
Using an itel imac
GH
The AHT must be run from the original installer DVD that came with your computer:
How to run hardware diagnostics for an Intel Mac
Boot from your original OS X Installer Disc One that came with your computer. After the chime press and hold down the "D" key until the diagnostic screen appears. Run the extended tests for a minimum of two or three hours. If any error messages appear note them down as you will need to report them to the service tech when you take the computer in for repair.
Some "common" error indicators:
SNS - sensor error
MEM - memory error
HDD - hard disk drive error
MOT - fan error